This invention relates generally to an electrophotographic printing machine, and more particularly concerns a color electrophotographic printing machine adapted to reproduce color transparencies.
The process of electrophotographic printing comprises exposing a charged photoconductive member to a light image of an original document. The irradiated areas of the photoconductive surface are discharged to record thereon an electrostatic latent image corresponding to the original document. A development system, thereupon, moves a developer mix of carrier granules and toner particles into contact with the photoconductive surface. The toner particles are attracted electrostatically from the carrier granules to the latent image forming a toner powder image thereon. Thereafter, the toner powder image is transferred to a sheet of support material. After the toner powder image has been transferred to the sheet of support material, the sheet of support material advances to a fuser which permanently affixes the toner powder image thereto.
The foregoing briefly describes the basic concept of electrophotographic printing. A wide variety of machines and devices have been developed for mechanization of this concept. The teachings of the prior art machines have, in the most part, been utilized to improve copies reproduced therein on a commercial basis. These improvements have been generally designed to solve a specific problem. Thus, for example, machines are presently in wide commercial use for reproducing microfilm. Machines of this type are described in U.S. Pat. No. 3,424,525 issued to Towers, el al. in 1969, U.S. Pat. No. 3,542,468 issued to Blow, Jr., in 1970 and U.S. Pat. No. 3,547,533 issued to Stokes, et al. in 1970.
In machines of the foregoing type, an enlarged copy of a microfilm input is reproduced. However, in all of the foregoing machines, it is extremely difficult to form reproductions of transparencies having photographic quality. Furthermore, no machines have been developed to produce photographic quality color copies from color transparencies, such as 35mm slides.
With the advent of color electrophotographic printing, it has become highly desirable to reproduce color transparencies as pictorial quality color opaque copies. Essentially, multicolor printing repeats the process of black and white copying a plurality of cycles, each cycle being for a different color. By way of example, the light image is filtered to record an electrostatic latent image on a photoconductive surface corresponding to a single color in the original document. The single color electrostatic latent image is then developed with toner particles complementary in color to the filtered light image. The toner powder image is then transferred to a sheet of support material. The foregoing process is repeated for successively differently colored light images. In this manner, a plurality of toner powder images are transferred to the sheet of support material, in superimposed registration with one another. Each of the toner powder images are complementary in color to the color of the filter utilized to produce the light image projected onto the photoconductive member. After a plurality of toner powder images have been transferred to the sheet of support material in superimposed registration with one another, the multi-layered toner powder image is permanently affixed thereto. The foregoing process is more fully described in U.S. Pat. No. 3,799,668 issued to McVeigh in 1973.
In color electrophotographic printing machines, the original document disposed upon a transparent platen is scanned to form a flowing light image thereof. Frequently, it is desirable to place a color transparency rather than a colored opaque copy on the platen as an original document. However, it has been found that the illumination system of the printing machine does not possess sufficient intensity. Light rays cannot pass through the image and reflect from the platen cover back through the transparency onto the photoconductive surface. Thus, it has not been feasible to reproduce color transparencies on a color electrophotographic printing machine.
Accordingly, it is a primary object of the present invention to improve color electrophotographic printing machines so as to enable color transparencies to be reproduced thereon.